This application is for support of a project focused on the acid-base physiology of selected bacteria from dental plaque, specifically Lactobacillus casei, Streptococcus mutans, Streptococcus sanguis and Actinomyces viscosus. These organisms were chosen to represent a wide range of acid tolerance from the highly tolerant L. casei to the relatively intolerant A. viscosus. A basic premise of the project is that acid tolerance is directly related to cariogenicity. To be cariogenic, a plaque bacterium must first colonize and become a significant member of the flora. It must then produce acid rapidly from dietary or salivary, sugars. It must be able to carry out glycolysis at the low pH values in plaque that lead to demineralization. The minimum pH value in cariogenic plaque is somewhat less than 4, and it is only the more acid-tolerant organisms that are active in this acid environment, especially the mutans streptococci, but also Lactobacillus organisms. The challenge to tooth integrity is related to the total time of acidification and to the extent of acidification during the acid-base cycles of plaque. An important part of the proposal concerns the abilities of cariogenic bacteria to adapt to acid environments and the mechanisms of adaptation. Preliminary results indicate that phenotypic adaptation does occur in chemostat culture and involves increased activity of F1F0 proton translocating ATPases. Adaptation results in acquired ability to carry out glycolysis at lower pH values. Thus, not only is acid tolerance a cariogenic attribute, but ability to adapt to acid environments may enhance cariogenicity. The major player in acid tolerance appears to be the FlF0 ATPase of cell membranes, and much of the proposed work is on basic enzymology, including isolation and purification of Fl and F0 sectors, separation of sector components, reconstitution of enzymes on membranes, in solution and in liposomes. Work is planned also on secondary systems involved in proton currents across the membrane, especially potassium transporters and proton-organic-acid symporters. The net result of the proposed work and related genetic work will be a clearer picture of the acid-base physiology of cariogenic plaque bacteria, and hopefully, ways to reduce the cariogenicity of plaque.